What's happening to us?

I met this guy in school last year and we got really close. But this year he's doing a work/apprenticeship, so he's not at school. No big deal, I can still text him and see him every once in a while. But lately he's acting strange. Like won't text me back or just really lame replys. I don't understand why and I care about him too much to just let him go. What should I do?